spunker88 said:
How do they prevent people with the very basic cable package (first dozen channels or so) from getting access to the other analog cable channels? What would prevent them from simply moving the digital versions of these channels near their former analog positions.
They use physical traps, little cylindrical filters (many of them made nearby in Syracuse at Microwave Filter Co.) that allow only certain RF frequencies to get through to the subscriber's drop. In the case of your TWC system, and mine, that basically amounts to a bandpass filter that lets through 2-6 (54-88 MHz), 7-13 (174-216 MHz) and a handful of frequencies in between. (Cable channels 14-22 are located just below channel 7, in the 120-174 MHz spectrum that's used for aviation, weather and two-way services in the world outside the cable drop.)
As you note, the filters stop around channel 70 or so, thus allowing QAM services on even higher RF channels to pass through.
It is indeed physically possible to put QAM multiplexes lower down on a system's bandwidth. On our system in Rochester, RF channel 5 (76-82 MHz) has, among other things, a couple of clear QAM services that carry the local public access channels.
If the cable company can filter out these analog UHF channels but leave others, whats stopping them from filtering out digital channels that are placed on these same UHF channels. I guess I dont get how they can filter out some channels and leave others.
I hope my explanation above helped to answer this question. They use physical traps, and physical traps have problems: they cost money to manufacture (especially more complex ones), they cost time and effort to install, they limit a system's flexibility (RF channel 14 still isn't used on our Time Warner system here because there are thousands upon thousands of 14-only traps on poles around town dating to the days when HBO analog lived on that channel and was trapped out of non-subscribers' homes), and they can easily be removed by would-be cable thieves. (I knew lots of people growing up who'd climbed the ladder behind the house to remove the 14 trap for free HBO, which was great until the cable guy showed up to do work on the lines.)
QAM digital gives a system huge flexibility to dynamically alter its spectrum use as needs change. As long as my cable box will map NBC to "channel 10," I don't ever need to know what "real" physical RF channel it's tuning to on the cable system. WHEC might be on QAM channel 100.9 today, but if TWC needs to shift it to 82.5 tomorrow, as long as my box knows to map it there, it doesn't matter to me. But if the cable company needed to roll trucks to remove or replace hundreds of thousands of physical traps to accommodate that change...well, they wouldn't. And that might mean there's not bandwidth available as a result to stick TruTV up in HD, or to increase my cable modem's downstream speed.
And it gets even more complex: many of those HD channels on Time Warner aren't even provided on fixed QAM frequencies. In order to provide the 100-some HD services it now offers, many less-viewed channels now come in via something called "SDV," switched digital video. If I want to watch Comedy Central HD, I punch "1166" into my box, and instead of tuning to a fixed QAM channel that translates to 1166, it sends a command to the cable headend saying I want to watch Comedy Central. The headend (or a device somewhere in my neighborhood between me and the headend) then dynamically assigns a QAM channel to Comedy Central and tells my box where to tune. When I watch the Daily Show tonight, it might be on 124.15...and tomorrow night, it might be on 123.8, and the night after that, it might be on 104.11. Again, as long as my box knows where to go, it's transparent to me, and I get dozens of channels in HD that I might not get otherwise.
(When too many people all want to watch different SDV channels at once, sometimes there aren't enough QAM channels available, which is why you occasionally see "Channel not available." They've done a pretty good job of avoiding that, though.)
But now imagine trying to make physical traps work with
that system! How can you trap out the channels you're not supposed to get, when those channels could show up anywhere in the RF spectrum?
Now, in the case of fiber-to-the-home systems like FiOS, my understanding is that all the RF that moves down the cables within the home is generated right at the box where the fiber enters the home. In a system like that, it should (I think) be possible to unlock QAM for all the channels a particular subscriber is authorized to receive, allowing for the use of clear-QAM tuners built into TV sets and other devices. I don't think Verizon has it implemented that way, but sadly, I'm in Rochester and we're in Frontier territory, so I don't get the chance to find out firsthand.
